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About 20 000 nematode species are known and they are assumed to be only a tiny fraction of the true number. More than 700 species have so far been found in Sweden. Most nematodes are microscopic transparent animals often less than 1 mm long. However, animal parasites may be much larger (from several cm up to several metres). The body is covered by a flexible cuticle, which is shed four times during the growth. There are four bundles of longitudinal muscles. The fluid of the body cavity is under pressure, which works antagonistic to the muscles (hydrostatic sceleton). The nematodes have sinusoidal movements and the body takes support against particles and water films. In the body cavity the tubular gonads and the intestine are found. Special respiratory and circulatory organs are missing. The anterior end of the intestinal system (the pharynx or oesophagus) is a muscular structure often equipped with bulbs and valves. Many species have both males and females and internal fertilization. However, in the soil fauna several species lack males and reproduce as hermaphrodites or by parthenogenesis.
The mouthparts are adapted to different food sources, which among soil living forms can be roots, fungal hyphae, bacteria or other animals.
Soil nematodesare small, generally 0.3-1.5 mm long, transparent animals living in the water-filled parts of the pore system of the soil. In the same habitat also other groups of microscopic animals, viz. protozoans, rotifers and tardigrades occur together with microorganisms (bacteria and fungi) and fine roots. In the food web of the soil the nematodes are found in several positions. Some eat bacteria, some browse on fungi or fine roots, others are true root feeders and still others are predators on microscopic animals. The relative abundance of various species constitute a fauna structure, which is partly dependent on soil factors and climate. The fauna structure will change upon disturbances such as cultivation and acidification.
Cervidellus spitzbergensis is a terrestrial, bacterial feeding roundworm found on Spitzbergen. The drawing shows a female with arrows pointing at some body parts and organs. The anterior end is shown on the detail drawing. The appendages are lips and so called 'labial probolae', which probably are of importance for feeding.
Text: Björn Sohlenius (Bjorn.Sohlenius@nrm.se) and Sven Boström (Sven.Bostrom@nrm.se) Ill. Bibbi Mayrhofer.
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